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IC 


8934 



Bureau of Mines information Circular/1983 




A Dynamic Gas-Mixing System 



By C. R. Carpenter, J. E. Chilton, 
and G. H. Schnakenberg, Jr. 




UNITED STATES DEPARTMENT OF THE INTERIOR 



^^tU^vUili^/. ^MjUJiM ^ yit^'jX 



Information Circular)8934 



A Dynamic Gas-Mixing System 



By C. R. Carpenter, J. E. Chilton, 
and G. H. Schnalcenberg, Jr. 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 






<^5 



^0' 



% 



.3H 



This publication has been cataloged as follows: 



Carpenter, C. R. (Clarence R.) 

A dynamic gas-mixing system. 

(Information circular / Bureau of Mines ; 8934) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:8934. 

1. Gases. 2. Mixing. I. Chilton, J. E. II. Schnakenberger, George 
H. III. Title. IV. Series: Information circular (United States. Bureau 
of Mines) ; 8934. 



TN295.U4 [TP242] 622s [665.7] 83-600081 



CONTENTS 



Page 



h 



Abstract 1 

Introduction 2 

Background 2 

System design 3 

Basic concepts 3 

Detailed system description 8 

Mass flow controller description 13 

Flow rate calibration 13 

System operation and performance 16 

Dilution ratios 16 

Flow setting reproducibility 16 

Flow controller linearity 16 

Binary mixture preparation 19 

Conclusion 20 

Future plans 20 

Appendix A. — Suppliers ' addresses 21 

Appendix B, — Digital timer operation and construction 22 

Appendix C. — Gas-mixing system program for Texas Instruments SR52 calculator... 28 



ILLUSTRATIONS 



1 . Basic elements of the gas dilution system 4 

2 . Flow control and mixing elements of the gas dilution system 4 

3. Electrical elements of the flow system 4 

4 . Linear and loop mixing manifold flow patterns 5 

5. Four controllers and two manifolds create two gas blends simultaneously.. 6 

6. Flow controller solenoid valves simultaneously actuated to direct flow to 
alternate manifold. 7 

7 . System with manually operated three-way valves 8 

8. Location of controls and connections for the manifold console and the 
control console 9 

9. Parts identification and functional layout of the manifold console 9 

10. Functional diagram of the dynamic gas-mixing system 10 

1 1 . Electrical schematic of the gas-mixing system 12 

12. Functional representation of the electronic mass flow controller sensor 
and valve 13 

13. Calibration curve for controller 1 17 

14. Calibration curve for controller P 17 

15. Calibration curve for controller 2 17 

16. Calibration curve for controller 3 17 

17. Calibration curve for controller 4 18 

18. Calibration curve for controller 5 18 

19. Measured methane concentration versus desired values 19 

vSi B-1. Functional block diagram of the digital timer 23 

i B-2. Time base generation and selection schematic 23 

B-3. Control and gating logic schematic 24 

B-4 . Counters and display schematic 24 

vj B-5 . Soap bubble transit detection circuitry 26 

y1) B-6 . Timer power supply schematic 26 

B-7. Timing diagram of logic states of control line of the timer 27 

C-1 . User instructions for gas dilution mixing program 29 

C-2. Program listing for gas dilution mixing program 30 






Ji 



XI 



TABLES 



Page 



1 . Flow conversion factors 15 

2. Reproducibility of a flow setting 16 

3. Summary of the linear regression analysis for Tylan flow controllers 18 

4. Desired and obtained methane-air mixtures 19 





UNIT OF MEASURE 


ABBREVIATIONS USED 


IN THIS REPORT 


A 


ampere 


ml 


milliliter 


° C 


degree Celsius 


ml/min 


milliliter per minute 


cm 


centimeter 


msec 


millisecond 


ft 


foot 


yA 


microampere 


hr 


hour 


liF 


microfarad 


Hz 


hertz 


pm 


micrometer 


in 


inch 


ys 


microsecond 


in Hg 


inch of mercury 


yyF 


micromicrofarad 


k 


kilohm 


n 


ohm 


K 


kelvin 


pet 


percent 


kHz 


kilohertz 


ppm 


part per million 


1/min 


liter per minute 


psi 


pound per square inch 


M 


megohm 


sec 


second 


mb 


millibar 


V 


volt 


MHz 


megahertz 


vol-pct 


volume-percent 


min 


minute 







A DYNAMIC GAS.MIXING SYSTEM 

By C. R. Carpenter, J. E. Chilton, and G. H. Schnakenberg, Jr. 



ABSTRACT 

A dynamic gas-mixing system assembled by the Bureau of Mines for the 
generation of precise gas mixtures from sources of concentrated or pure 
gases and diluent gases is described. A set of electronic mass flow 
controllers with maximum flows ranging from 10 to 5,000 ml/min, a so- 
called Pure Air Generator, and gases in cylinders are used to generate 
differing gas concentrations. The repeatability of the delivery rate 
of a flow controller, measured on different days, has a precision of 
0.3 pet of the setting. This dynamic gas-mixing system reduces the 
cylinder inventory and the demurrage charges for special gas mixtures. 
Gas mixtures that cannot be shipped commercially, such as flammable 
mixtures of methane in air, can be conveniently prepared by this sys- 
tem. The maximum dilution ratio is 2.5 x 10^. Thus, gas mixtures can 
be made over a wide range of concentrations, from the percent region 
(by dilution of a pure single-component gas) to fractional parts per 
million (by dilution of a premixed standard, e.g., a 1,000-ppm mix- 
ture) . This system is especially useful in determining the response of 
gas detection devices over the entire range of their measurement. Be- 
cause the controllers are voltage controlled they lend themselves eas- 
ily to automated control using computer-based systems. 

^Electronics technician. 
^Research chemist. 
^Supervisory research physicist. 
All authors are with the Pittsburgh Research Center, Bureau of Mines, Pittsburgh, 
PA. 



INTRODUCTION 



There are many applications for gas de- 
tector devices for personal safety and 
property protection in the mining indus- 
try. The devices are used to detect the 
presence and concentration of (1) toxic 
gases (e.g., hydrogen sulfide), (2) gases 
that are precursors of fire (e.g., car- 
bon monoxide) , (3) gases that may accumu- 
late to form explosive mixtures (e.g., 
methane), and (4) life-threatening atmos- 
pheres (e.g., deficiency of oxygen). 
These gas detectors must be sensitive 
over a wide range of concentrations to 
selected species of gas. The performance 
of each type of detector should be evalu- 
ated prior to its general use to insure 
its adequacy and suitability. 

The evaluation of gas instrumentation 
designed to detect low concentrations 
(parts per million) of CO, NO, NO2 , SO2 , 
and H2S and high concentrations (volume- 
percent) of O2 , CH4 , and CO2 for use in 
mining environments requires gas mixtures 
of high accuracy for both instrument cal- 
ibration and instrument performance eval- 
uation. The performance evaluation of 
gas instrumentation will use well- 
characterized and accurately determined 
gas calibration mixtures for tests 



including those for accuracy-over-range, 
drift, and precision. 

Calibration gases may be purchased in 
cylinders from commercial sources but not 
without several disadvantages: high ini- 
tial cost; uncertainty in accuracy of 
analysis; slow, unpredictable delivery; a 
finite cylinder capacity; and extensive 
demurrage charges incurred in the storage 
of a large number of reference gas mix- 
tures in cylinders. Certain mixtures of 
gases may not be available because of 
their flanmability. One solution to this 
problem of obtaining different concentra- 
tions of a gas is to dilute a high- 
concentration gas accurately to a lower 
concentration, A dynamic gas dilution 
system involves the mixing of two gas 
streams to produce a predetermined spe- 
cific concentration of a gas in a flowing 
gas sample. 

This report describes the design and 
fabrication of a dynamic gas dilution 
system that satisfies the demands of gas 
instrument evaluation test methods that 
require a flowing source of several accu- 
rately known gas mixtures. 



BACKGROUND 



The Bureau of Mines instrumentation 
group at the Pittsburgh Research Center 
has used various methods in its attempts 
to design and build a universal dynamic 
gas-mixing system. One system employed 
precision needle valves to control the 
flows of the test gas and the diluent 
gas. The flow rates were monitored with 
high-quality Labcresf*'^ rotameters. 

Although this method was exclusively 
used for several years, it had several 
deficiencies. The main deficiency was 
its imprecision or lack of resetabillty; 

^Use of trade names is for identifica- 
tion only and does not imply endorsement 
by the Bureau of Mines. 

^Suppliers' addresses are listed in 
appendix A. 



each and every flow setting required ver- 
ification using a soap bubble meter to 
obtain desired mixing accuracies (±1 pet 
or less error) . Another deficiency was 
its lack of long-term stability. One 
source of this instability was the vari- 
ation in the regulation of pressure 
from the cylinder gases. At high dilu- 
tion ratios, a small change in input gas 
pressure of the minor component gas 
caused a large change in the diluted gas 
concentration. 

In an effort to improve our testing 
facilities, we experimented with restric- 
tors. We found the usable range of flow 
rates of each resistor to be limited 
owing to their nonlinearity . Additional- 
ly, the instability problem was not 
significantly improved even though 



additional pressure regulators were 
placed in the input lines. 

Our next major effort was centered on 
the use of electronic mass flow con- 
trollers (Tylan model 261). Preliminary 
tests on the devices indicated that they 
would be very suitable in our applica- 
tion. We had several reasons for select- 
ing electronic mass flow controllers 
(MFC): 

1. As flow controllers, the mass flow 
rate would be independent of the pressure 
upstream of the controller. 

2. The mass flow could be set by ap- 
plying a voltage between 0.1 and 5 V, and 
voltages of sufficient stability could be 
easily produced. Furthermore, the flow 
rate would be approximately proportional 
to the control voltage over the range of 
the controller. 

3. The controller provided an output 
signal of 0.1 to 5 V, also approximately 
proportional to the mass flow of the gas. 
This signal could be accurately read by a 



digital panel meter, and it is also equal 
to the flow rate control signal when con- 
trol of the flow was attained by the 
controller, 

4. The manufacturer claimed a preci- 
sion (resetability) of 0.2 pet of full 
scale, a precision which we thought nec- 
essary for our gas detection instrument 
investigations and evaluations. 

5. Once a calibration curve was estab- 
lished for a particular controller for 
one gas, a simple application of the gas 
laws and well-known physical properties 
of gases could be applied to this curve 
to produce a calibration curve for other 
gases or for other ambient pressures and 
temperatures. 

6. Since flows are voltage controlled, 
these controllers could be easily used in 
an automated testing facility should this 
be needed. The following sections de- 
scribe first the design concepts and 
rationale and then the full system design 
and performance of our dynamic gas dilu- 
tion system using mass flow controllers. 



SYSTEM DESIGN 



BASIC CONCEPTS 



The purpose of this gas-mixing system 
is to dilute a high concentration of test 
gas species to a more appropriate lower 
concentration for use in gas instrument 
testing, thus conserving gas and permit- 
ting a variety of concentrations from one 
gas cylinder. The inputs to the system 
are the one or more test gas species to 
be diluted (source gas) and the diluting 
gas (diluent) . The output is the diluted 
gas mixture. 

Because the output concentration is 
determined by flow rates, they become the 
critical elements in the system design. 
Figure 1 depicts the basic elements of 
the gas dilution system involving two 
components (source and diluent). The 
flow controllers are adjusted by the 
operator to produce the desired flow ra- 
tio and, therefore, the concentration of 
the blended gases. After mixing, the gas 



is conducted by suitable connectors to 
the system output. Figure 2 emphasizes 
the flow control and mixing elements of 
the system. 

The flow controllers control the flow 
rates of the gas in response to a command 
(set point) voltage. In our systems, 
this voltage, between 0.1 and 5 V, is ob- 
tained from a front panel control that is 
set by the operator (fig. 3). A voltage 
proportional to the flow rate is devel- 
oped by the flow controller. This volt- 
age, which, as previously mentioned, is 
equal to the command voltage when the 
flow is stabilized, is displayed by a 
digital panel meter (DPM) . As a refine- 
ment, since each controller has a DPM 
dedicated to it and since each covers 
different flow ranges, we have adjusted 
the calibration of each meter to provide 
a display that is approximately numeri- 
cally equal to the flow in milliliters 
per minute. Since it is necessary for 




Flow 



(fi) 





Flow 2 (fg) 


Mixing 


Total flow (ft) 




chamber 












Diluent 
gas 













Diluted 
concentration 

out (Cq) 



FIGURE 1, - Basic elements of the gas dilution system: source of a minor component at concentration C; 
flowing at rate f ^; a diluent gas, e.g., air flowing at rate f2; and a mixing element. The final concentration 
of the mix is C. x f i/ff flowing at a rate of f^ = f ^ + fj. 



Gas to be 
diluted 



Diluting 
gas 




Diluted 
*" gas 
output 



Mixer-manifold 



FIGURE 2. - Flow control and mixing elements of the gas dilution system. 



Signal 
out 



Moss flow 
controller 



Connnnand 
voltage in 



Press for input 




Scale 
factor 



Adjust 



Digital panel meter 



Set point C"? vdc 
(command) 



0.1 Vdc 



FIGURE 3. - Electrical elements of the flow system. Each mass flow controller has associated with it a 
power supply (not shown) and a command signal to be set to the desired value by the user. A digital panel 
meter is appropriately scaled to read flow rate directly in milliliter per minute and is used to display eith- 
er the command signal to the controller or (normally) the flow rate signal output from the controller. 



the operator to see the flow setting com- 
mand voltage to set the particular con- 
troller, this voltage can be displayed on 
the same digital display, using a momen- 
tary contact pushbutton switch. 



The output flows of the controllers 
should be mixed thoroughly and be avail- 
able with a minimum response (lag) time. 
To accomplish thorough mixing, the 




"o 



o 
o 

•a 



^ Stagnant ^{ ^ 
{^ region ^ ^' 



o 

Q. 

E 
o 
o 



<1> 

c 
o — . 

C 0) 

8.-2 
o "^ 



Low-flow 
region 



"High-flow 
1 region 



ZOul 



Linear manifold 



Major component 

(diluent) ^r^ 



Minor 
component 



Unused 
controllers 




-Out 



Closed-loop manifold 

FIGURE 4. - Linear and loop mixing manifold patterns. 



manifold into which the controllers de- 
liver the gases should have no stagnant 
areas that could contain unmixed gases 
that would slowly diffuse into the mix- 
ture. To minimize response time, the to- 
tal volume must be kept small. The stag- 
nant volumes are substantially eliminated 
by having all controllers feed into a 
closed-loop manifold. When a number of 
controllers are connected together in a 
linear deadend manifold, the length of 
tube beyond the entrance point of the ac- 
tive controller farthest from the mani- 
fold outlet would contain stagnant vol- 
umes of gas of unknown composition 
(fig. 4). Furthermore, if this control- 
ler was delivering the minor component at 
a low flow, it would require a signifi- 
cant time to flush the section of the 
manifold between it and the entrance 
point of the major component. Creating a 
loop manifold by connecting the ends of a 
linear manifold to each other and exiting 
the manifold through a tee, as shown in 
figure 2, causes the flow from the high- 
est flowrate controller to split and 



sweep the entire manifold at relatively 
high flows, picking up and mixing with 
the minor component in the process. 

An internal volume of about 2 ml in the 
manifold is realized by employing stan- 
dard 1/4-in tubing for its fabrication; 
this keeps the response time small. Tur- 
bulence of the gas caused by 90" bends 
and the fittings in the closed loop in- 
sures complete mixing of the gases. 

The addition of a second mixer- 
manifold, the two three-way valves, and 
another output connector increases the 
operating flexibility of the system (fig. 
5) . The two manifolds are designated as 
A and B. The need for this added flexi- 
bility is described in the following 
example. 

Suppose the operator wanted to chal- 
lenge a gas detection instrument with a 
test gas that rapidly changes 
concentration — a step change. To do this 
the operator would set up a gas mixture 



Inputs 




output B 
5 and 4 
detector) 



Dual mixer-nnanifold 

FIGURE 5. - Four controllers and two manifolds create two gas blends simultaneously. Solenoid 
valves for controllers 1 and 2 direct the gases into manifold A, with controller 1 having the larger 
of the two flows. Similarly controllers 3 and 4 feed manifold B, with controller 3 having the great- 
er flow. Compare output blends with those in figure 6. 



in manifold B using the flow controllers 
3 and 4 in figure 5. The solenoid 
valves, 3 and 4, associated with these 
controllers would be set to deliver the 
flows to manifold B, The gas detection 
instrument would be connected to mani- 
fold B. Next, using flow controllers 1 
and 2 with their solenoid valves set to 
deliver the flow to manifold A, the oper- 
ator would establish a different mixture 
in manifold A and route the output to a 
vent. To deliver a step change in gas 
concentration to the gas detection 
instrument, the operator must simultane- 
ously switch all four solenoid valves, 
whereupon controllers 3 and 4 would de- 
liver flows not to B but to A, and con- 
trollers 1 and 2 would deliver flows not 
to A but to B, as shown in figure 6, 
This, in effect, would challenge the de- 
vice under test with a "step function" of 
the test gas. This "step function," 
among other things , would be useful in 
determining the response time of the gas 
instrument being tested. Rapid switching 
of all of the flow controllers between 



manifolds A and B is assured by using 
three-way electrical solenoid valves at 
the output of each flow controller. Si- 
multaneous switching of all valves is ac- 
complished using a single switch. 

A Swaglok manually operated three-way 
valve located at the output of each con- 
troller provides additional flexibility 
to the system, as shown in figure 7. One 
position of this valve directs the gas to 
the solenoid manifolds selection valve 
described above. The other position di- 
rects the gas from the controller to an 
associated auxiliary output port. These 
auxiliary outputs provide the user with 
an undiluted gas at a controlled flow 
rate. The valve may also be used to cut 
off the gas flow by setting them to a 
midposition. 

The source gases and the nitrogen dilu- 
ent gas are usually obtained from cylin- 
ders. These gases are delivered from a 
cylinder storage area to the gas-mixing 
system by separate stainless steel tubes 



Inputs 




output B 
I and 2 
detector) 



Dual mixer-manifold 

FIGURE 6. - Flow controller solenoid valves simultaneously actuated (energized or deenergized 
depending on original state) to direct flow to alternate manifold. Thus, at manifold A the gas mix 
rapidly changed from a blend of 1 and 2 to a blend of 3 and 4; similarly the output from B changed 
from a blend of 3 and 4 to a blend of 1 and 2. Compare with figure 5. 




■,—r. Gas 
HID output 

B 

(~ii Gas output 
A 



Manual 

selector 

valve I 

Dual mixer-manifolds 

FIGURE 7. - System with manually operated three-v^/ay valves. 



from each cylinder. The pressure is reg- 
ulated to the operating input pressure 
range (10 to 40 psi differential) of the 
controllers . Some gases , such as NO2 , 
H2S, and SO2 , are conveniently obtained 
from a permeation tube system. These 
gases are normally diluted in a fume hood 
with the diluent gas controlled by the 
gas-mixing system; this is an ideal use 
of the auxiliary outputs. Air, when it 
is used as the diluent gas, is supplied 
from a commercially available generator 
of pure (zero) air. This air is made 
available to the gas-mixing system 
through a separate line. 

DETAILED SYSTEM DESCRIPTION 

The mixing system consists of two 
units , a manifold console and a control 
console (fig. 8), Three auxiliary sys- 
tems are routinely used with the dynamic 
gas-mixing system: a cylinder manifold, 
a commercially available Pure Air Gener- 
ator, and a permeation system. 

The manifold console (fig. 9) consists 
of two closed-loop manifolds (CLM-A and 
CLM-B) , six mass flow controllers (MFC) , 
six manual selector valves (MSV) , and six 
three-way solenoid valves (SV). Six 
stainless steel filters, 7-ym pore size, 
are located outside the console for easy 
replacement and are placed upstream of 
the MFC to prevent aerosols from entering 
the system. 



The gases to be mixed are connected to 
the system by compression seal bulkhead 
fittings on the top of the console, with 
one corresponding to each controller. 
Each gas flows through the filter and 
then directly to a mass flow controller. 
Immediately downstream of each controller 
is a manual selector valve that is used 
to route the gas to either an auxiliary 
output port or an associated solenoid 
valve which, in turn, routes the gas flow 
to one of two output manifolds (labeled 
CLM-A or CLM-B). With the manual selec- 
tor valve knob in the center position 
(horizontal) , the gas flow through that 
channel is cut off. 

The auxiliary outputs, when selected, 
allow the precisely controlled flow to be 
accessed directly when mixing is not re- 
quired or when it is to be used as a 
source for associated equipment, i.e., as 
the dilution gas for a permeation tube 
system. 

When gas mixing is desired, the manual 
selector valves associated with the se- 
lected controllers are positioned to di- 
rect the flow of the gases to the associ- 
ated solenoid valve. With the solenoid 
valves deenergized, the gases flow 
through CLM-B. In the manifold the gases 
are mixed and presented to the front pan- 
el output port labeled B. When a sole- 
noid valve is energized, the controlled 
flow from its associated controller 



p 


O O O O 


o. 


Gas inputs 
Top panel (detail) 
Auxiliary outputs 


O 


O O O O 


^ 

o 



Front 



12 3 4 
Manual selector valves 



Hood 
vent 

output ^ g 



Manifold 



Q Manifold 
g output 



DPM-P 



DPM-I 



DPM-2 



DPM-3 



DPM-4 DPM-5 



Po lo 2o 3o 4o 5o 

vPB-l /?=xPB-2 ^^=nPB-3 /?=^PB-4 /^PB-5 /?=^PB- 

OOOOOOOOOOOO 

S-l S-7 S-2 S-8 S-3 S-9 S-4 S-IO S-5 S-ll S-6 S-12 



Manifold control panel 



o 9 

S-14 I- 1 



O 

S-13 



Manifold console Control console 

FIGURE 8. - Location of controls and connections for the manifold console and the control 
console. 



Top panel 



r 

- — D— 



Out I 

— D— 



In 



Out 2 



Out 3 



— D— 



Out 4 



-D— 



Out 5 

— D-H 



In 



MSV 
positions 

Auxiliary 
Monifold 




Front panel 

LEGEND 
* 7-^m replaceable filters 

FIGURE 9. - Parts identification and functional layout of the manifold console. 



10 



Auxiliary 
gas outputs 



Gas inputs 



MSV< 



CLM-A 





[ H 




XLM-B 



'_ 




MFC-4 



MFC -3 



MFC-2 



MFC-I 



MFC-P 



r~r 



Fume 
hood 



I I 
I I 



j lPPMl 



fo 



PB-2 



iDPMl 



T-O 



,PB-3 
IDPMI 



*-o 



I 

P 



PB-4 



iDPMl 



1 — 1 

I 

|PB-5 



PPM 



I [— 



fO 



fpB-6 



PPM 



ADJ 



rVsAn nAAn r\AAn nAAn iVsAn iWV> 

■ "-^^ I i_ADJ_i. I ADJ__i |J\Dm I J^DJ_i. |.ADJ_i 

,_2____^_3____^.i _^ 5 ^ 

5 Vdc 



_ Flow control _ 

Manifold control 
, _6^ I ^ ^ ^ 

T Normal 6^ ^ , 

LReyers_e_Ps.|3 So^^ °II7 Vac 

Control console 

FIGURE 10. - Functional diagram of the dynamic gas-mixing system. The control console is con- 
tained within the dashed-line box; the manifold console contains the remaining parts of the diagram. 



12 



l_. 



enters CLM-A, The system is designed so 
that the solenoid valves are individually 
energized and deenergized. This feature 
adds flexibility to the system (fig. 10). 

The manifolds are constructed from 
standard compression fittings and lengths 
of 1/4-in stainless steel tubing. The 
closed-loop manifold design eliminates 
dead zones and thus minimizes the volume 
of pockets of unmixed gas that would 
slowly diffuse into the main stream. The 
internal volume of each manifold is kept 
to a minimum (approximately 2 ml) . 

A stainless steel tube from an adjacent 
fume hood is brought out to the front 



panel of the console and is terminated 
with a tubing-to-hose connector fitting. 
This fitting is marked "H" on the panel. 
This line is used to carry test gases to 
the hood. 

The control console is divided, func- 
tionally, into two sections, the flow 
rate control and manifold control, as in 
figure 8. The sections are electrically 
connected using jack J-1 and plug P-1. 
The control section (refer to figs. 8 and 
10) has six miniature toggle switches, 
S-7 through S-12, that are used to ener- 
gize (and deenergize) the individual 
solenoids that select the output CLM-A or 
CLM-B for each controller in the manifold 



11 



console. Associated with these switches 
are two control switches. One switch, 
S-13, is designated "NORMAL/ RE SERVE" on 
the panel. The purpose of this switch is 
to produce a rapid change of all control- 
ler outputs from one manifold to the oth- 
er by simultaneously switching the state 
of each solenoid valve. 

Having two manifolds also permits two 
operators to use the mixing system. That 
is, one operator can use manifold A while 
the other operator uses manifold B, pro- 
vided there is no conflict in the use of 
individual controllers. The most used 
controllers in our laboratory have been 
the 4- to 200-ml/min and the 40- to 
2,000-ml/min controllers. Our system in- 
corporates two of each. Therefore, this 
needed flexibility is usually available. 
In addition, of course, the AUX outputs 
can be used simultaneously and blended 
external to the system. 

The second switch, S-14 (POWER/OFF), is 
a master on-off switch for the solenoids, 
A neon panel lamp (I-l) illuminates when 
the solenoid power is on. 

The remainder of this console is de- 
voted to the control of the gas flow 
rates. Each flow controller or gas com- 
ponent "channel" is laid out vertically 
on the panel. Each channel includes a 
digital panel meter, a potentiometer for 
setting the flow-controlling (command) 
voltage, the switch that effects a momen- 
tary display of the control voltage on 
the panel meter, and the manifold se- 
lection switch described previously 
(fig. 8), 

In our system the flow channels labeled 
"P" and "1" through "4" use 3-1/2-digit, 
digital panel meters (DPM) to indicate 
the gas flow rates, Channel 5 employs a 
4-1/2-digit meter since this is a 100- 
to 5,000-ml/min controller and requires 
a display resolution beyond that pro- 
vided by a 3-1/2-digit meter. Potentio- 
meters at the inputs (pin 1) of the panel 
meters (fig, 11) form voltage dividers 
which allow calibration of the read- 
outs to be equal to the flow rate in 



milliliters per minute. The range of the 
3-1/2-digit meters is 0.000 to 1.999 V. 

The most significant digit of the 
4-1/2-digit DPM (that is, the "1" of the 
19999 maximum display) is not used. This 
gives a usable display of the digits up 
to 9999, as opposed to 1999 of a 
3-1/2-digit meter. As mentioned earlier, 
the full-scale signal output for maximum 
flow of the controllers is 5 V regardless 
of the full-scale range of the control- 
ler. The potentiometers at the panel me- 
ter inputs allow adjustment for an exact 
panel meter indication, directly in mil- 
liliters per minute, of the rate of gas 
flow through the channel at one flow set- 
ting. In our system, we set the meter to 
agree with the flow near the full-flow 
rating of the controller in each channel. 
Subsequent checks at various flow rates 
within the range of each controller were 
performed and used to develop a set of 
calibration graphs (figs, 13-18) that re- 
late the controller output voltage to ac- 
tual measured gas flow. All values were 
corrected to our reference standard tem- 
perature and pressure (25° C and 28,92 
in Hg), Using these graphs and tempera- 
ture and atmospheric pressure correction 
factors , the operator can routinely ob- 
tain mixing accuracies with less than 
5 pet error. When greater accuracy is 
required, the flow rates are verified by 
the use of a soap-bubble meter. 

The operator sets the flow rates by ad- 
justing the set-point potentiometer (R-1 
through R-6) in the desired channel 
(figs, 8 and 11), To facilitate this ad- 
justment, the controller input signals 
(set points) may be observed on the panel 
meters (in milliliters per minute) by de- 
pressing the PRESS FOR INPUT pushbutton 
switches (PB-1 through PB-6) . 

Each controller is energized or deener- 
gized independently by operating its 
POWER-ON/OFF switch (S-1 through S-6) . 
The power switches supply 115-Vac power 
to the operational power supplies, P.S.-2 
and P.S,-1, (±15-Vdc and 5-Vdc reference 
supplies) and the panel meters. The me- 
ter associated with channel 5 uses an 



12 



LEGEND 
» Press for input 



Manifold 
console 




FIGURE 11. - Electrical schematic of the gas-mixing system. 



13 



external 5-Vdc (P.S.-3) supply; the other 
meters are powered directly from the 
switched 115-Vac line. The output from 
the 5-V reference supply is permanently 
connected to the clockwise end of all six 
set-point (command) potentiometers. The 
±15 Vdc is connected to the individ- 
ual controllers through separate poles 
of the power switches . Thus only the 
controllers in use are powered, saving 
wear and tear on any unused controllers. 

MASS FLOW CONTROLLER DESCRIPTION 



resistance temperature sensors and heat- 
ers. This is shown in figure 12. A more 
complete explanation can be found in the 
Tylan manual. 

FLOW RATE CALIBRATION 

Calibration of the system was performed 
using zero air produced by a Pure 
Air Generator, a soap-bubble flowmeter 
set, an electronic timer, an electronic 
digital thermometer, and an aneroid 
barometer. 



The electronic mass flow controllers 
used in this system accurately and reli- 
ably measure and control the mass flow of 
gases. The principles of heat transfer 
along a capillary tube are used to devel- 
op a linear output signal of 0. 1 to 5 V 
over a selected flow range. The control- 
lers incorporate a valve and appropriate 
electronics to automatically regulate 
flow rates in response to an external 
command signal (0.1 to 5 V). 

The valve (fig. 12) , made of 316 stain- 
less steel, is a unique thermal expansion 
design that eliminates friction and 
moving seals . It is a small thin-walled 
tube with a ball welded to the end. The 
seat is a cone. Inside the tube is a 
heater wire that causes the tube to ex- 
pand relative to the outer shell, moving 
the ball relative to its seat and thereby 
varying the flow. 

The sensor section consists of a small, 
stainless steel capillary with external 



The Pure Air Generator, according to 
the manufacturer (AADCO) , produces air 
with less than 0.005 ppm hydrocarbons, 
carbon monoxide, carbon dioxide, methane, 
ozone, sulfur, hydrogen sulfide, and ox- 
ides of nitrogen but with 22.5 pet O2 in 
nitrogen. A methane reactor accessory, 
mounted within the cabinet, is a low- 
temperature catalytic oxidizer which re- 
moves all combustible hydrocarbons, in- 
cluding methane. The air compressor for 
this unit is capable of supplying up to 
10 1/min of zero-air and is mounted in a 
housing to reduce noise to an acceptable 
level. 

The Teledyne soap-bubble flowmeter set 
contains a set of three glass tubes of a 
calibrated volume and traceable to the 
National Bureau of Standards (NBS). Each 
tube has a different volume (10, 100, and 
1,000 ml) and fits in a glass base which 
contains a soap solution such as those 
available for detecting gas leaks. (We 
use SNOOP.) The gas is directed into the 



Sensor assembly 



,lr/jj 'm 




^^^^ 



Bypass assembly 



Gas 



out 



Valve assembly 



FIGURE 12. - Functional representation of the electronic mass flow controller sensor and valve. 



14 



glass base and passes out through the 
graduated tube. A squeeze bulb, fitted 
on the base, is used to raise the level 
of the solution in the base to block the 
entrance to the graduated tube. When the 
bulb is released, the liquid level drops; 
a film of the solution remains across the 
flow passage and moves up the graduated 
tube as it is pushed by the flowing gas. 
The flow rate is determined by measuring 
the transit time for the film to pass 
through a given volume. That is, 



flow rate = 



volume (ml) 



transit time (min) 



Corrections for temperature, atmospheric 
pressure, and gas composition if other 
than air are then applied to obtain the 
mass flow rate (i.e., a volume flow rate 
referenced to a given pressure and tem- 
perature) . Because our laboratory is 
slightly greater than 1,000 ft above sea 
level, and operating equipment keeps it 
warm, we standardize at 28.92 in Hg 
(979.3 mbar) and 25° C. 

To improve precision and eliminate the 
random operator errors in measuring bub- 
ble transit times, we developed a digital 
timer (appendix B) for which the start 
and stop signals are obtained from two 
phototransistors which detect the pas- 
sage of the moving film (bubble) that 
is illuminated from above. ^ The first 
phototransistor is positioned at the bot- 
tom or zero graduation on the tube. As 
the edge of the film passes, it acts as a 
"light pipe" that momentarily increases 
the light transmission to the phototran- 
sistor. This signal starts the timer. 
As the film passes the second phototran- 
sistor, positioned at the top graduation 
(10, 100, 1,000 ml) of the tube in use, a 
stop signal is similarly generated that 
stops the timer. 

Comparison between a series of soap- 
bubble transit times measured manually 

^No commercial bubble transient timer 
was available at the time of our need. 
Presently, we understand such a device is 
commercially available from at least one 
source (Mast) . 



and those simultaneously measured by 
the phototransistor-timer verified the 
accuracy of the phototransistor separa- 
tion and the increased precision obtain- 
able with the timer. 

For a particular measurement a tube is 
selected to provide a sufficient transit 
time to insure adequate resolution at the 
flow rate being measured. 

The timer, designed and constructed 
specifically for this job, incorporates 
other features that allow its use as 
a general purpose laboratory timer. A 
block diagram and schematics for the 
timer are included in this paper as ap- 
pendix B. For use with the soap-bubble 
flowmeters, the 0- to 10-min range is 
normally selected. This gives a resolu- 
tion of +0.0001 min for times up to 
9.9999 min. 

For gas temperature measurements, we 
use an electronic digital thermometer 
from Stow Laboratories. The platinum re- 
sistance sensor is attached to the gradu- 
ated tube of the flowmeter to measure, 
indirectly, the ambient temperature, and 
hence, the temperature of the gas. The 
digital display on this unit provides a 
resolution of +0.1° C. 

Barometric pressure is measured on 
either an aneroid barometer or an elec- 
tronic meter; the aneroid barometer is 
an analog device, and resolution of 0.01 
in Hg is obtainable. The Serta Systems 
electronic meter resolves to 0.1 mbar. 

A short calculator program was devel- 
oped to expedite the calibration of the 
gas-mixing system. The program (appendix 
C) was written for the Texas Instruments 
(TI) SR-52 programmable calculator and 
can be directly entered in the newer TI 
58 and TI 59 calculators by replacing the 
HLT commands with R/S. For calculators 
that do not use the Algebraic Operating 
System, the equation is 



fc = 



Ts 



Pa 



fi 



Ta • Ps 



(1) 



15 



where fc = corrected flow rate, ml/min, 

Ts = standardized temperature 
(25° C + 273.15), K, 

Pa = actual barometric pressure, 
Hg or mbar, 

Ta = actual temperature (° C 
+ 273.15), K, 

Ps = standardized barometric 

pressure (28.92 in Hg or 
979.3 mbar). 



and fg = flow rate of other gas, 
ml/min. 

TABLE 1. - Flow conversion factors 



and 



fi = indicated flow rate, ml/min. 



The permeation of the gas through the 
soap bubble did not cause any significant 
error in our flow measurements with 
transit times of less than 2 min. 

The flow controllers, as received, were 
calibrated for air by the manufacturer. 
One must use a correction factor to use 
the controllers with gases that have dif- 
ferent molecular structure. In their 
Operation and Service Manual, Tylan Corp. 
provided a table of correction factors 
for various species of gas. We conducted 
experiments in the laboratory to deter- 
mine the corrections required for meth- 
ane, carbon monoxide, and carbon dioxide. 
The values obtained were in very close 
agreement with the values published in 
the Tylan table. 

Typical examples of this correlation 
are shown in table 1. Flow controller 2 
was set during each gas measurement to an 
indicated flow rate of 150 ml/min (on as- 
sociated panel meter). The bubble meter 
tube volume was 100 ml. The flow rates 
shown are corrected to our standard tem- 
perature (25° C) and pressure (28.92 in 
Hg). The equation for the conversion 
factors is 



r - 9, 



where Cf = conversion factor, 

fa = flow rate of air, ml/min. 





Manufacturer's 


Experimental 


Measured 


Gas 


conversion 


conversion 


flow 




factor 


factor 


rate, 
ml/min 


Air 


1.00 


1.00 


147.1 


CH4 


.72 


.72 


105.9 


CO 


1.00 


.99 


146.1 


CO2 


.74 


.74 


108.9 



Most of the test gases used in our 
testing programs are obtained from cylin- 
ders that are located remotely from the 
dynamic gas-mixing system. We have found 
it undesirable to store some gases in 
cylinders (NO2 SO2 , H2S, etc.) because of 
reaction with the cylinder walls (unsta- 
ble concentration) or toxicity of the 
gas. For generating mixtures of these 
gases, we use a permeation tube system. 

Our permeation system uses a Forma Sci- 
entific water bath in which the tempera- 
ture is held constant at 20° ±0.1° C by an 
integral refrigeration and heating sys- 
tem. Two Pyrex glass permeation tube 
holders (U-tubes) are immersed in the wa- 
ter bath. This allows us to generate two 
species of test gas simultaneously. We 
use 10-cm-long permeation tubes , which 
gives an active tube life of more than 12 
weeks at the operating temperature. Dry 
nitrogen is used as the carrier gas for 
these tubes at a flow rate of 100 ml/min 
for each tube holder. The two outputs 
from the system are brought out inside a 
fume hood which vents the gases when they 
are not in use. Under the operating con- 
ditions , the output concentrations are 
relatively high (for NO2 , ~75 ppm) and 
the gases are normally diluted to usable 
levels by mixing with air or nitrogen 
from the flow control system. 

The output concentration from this sys- 
tem is determined gravimetrically by mea- 
suring the weight loss of the permeation 
tube every few days. The short-term and 
running-average weight losses are com- 
puted, recorded, and graphed. 



16 



SYSTEM OPERATION AND PERFORMANCE 



DILUTION RATIOS 

The concentrations of test gas required 
by the instrument evaluation laboratory 
range from high percentage to low parts 
per million levels, a range of 1,000,000 
to 1. As a compromise the design goal of 
a dilution ratio of 1:1,000 was chosen. 
This will produce a test gas concentra- 
tion of 0.1 pet (1,000 ppm) from a source 
of pure gas or, if a cylinder of accu- 
rately known premixed gas (e.g., a 
1,000-ppm concentration) is used as the 
source, low-part s-per-million gas concen- 
tration levels can be achieved (i.e., 
1,000 ppm/ 1,000 = 1 ppm). Of course, in- 
termediate concentrations can be obtained 
by adjusting the dilution ratio; there- 
fore, any test gas concentration between 
1 ppm and 100 pet is available for our 
gas detection device evaluation efforts. 
This satisfies our flexibility require- 
ments and reduces the cylinder gas inven- 
tory to a maximum of two cylinders for 
each species of gas required for testing. 
In our experience gas detection devices 
require flows of up to approximately 1 
1/min, To test several devices from a 
common source, flows up to 5 1/min might 
be required. Therefore, for 1:1,000 di- 
lution ratio, we selected four flow rate 
ranges for our system controllers to cov- 
er this dilution range at the desired 
output flow rates: 0.2 to 10, 4 to 200, 
40 to 2,000, and 100 to 5,000 ml/min. 
These ranges allow the total output flow 
rates to be adjusted, as required for 
testing, between 200 ml/min and 5 1/min 
at the maximum dilution ratio. The maxi- 
mum dilution ratio is thus 0.2 to 5,000, 
or 1 to 25,000. 

FLOW SETTING REPRODUCIBILITY 

The reproducibility of setting the gas 
flow of the Tylan mass flow controller 
was calculated by setting a fixed flow 
value on a controller and periodically 
measuring the resulting gas flow on dif- 
ferent days over a period of several 
weeks . Since the measurements were made 
on different days and thus at different 
ambient temperatures and barometric 



pressures , the daily average flow read- 
ings were corrected from measured temper- 
ature and pressure to our chosen refer- 
ence temperature and pressure of 25° C 
and 28.92 in Hg (979.3 mbar) . This value 
of the pressure corresponds to normal 
atmospheric pressure 29.92 in Hg or 
(1,013.2 mbar) corrected for a 1,000-ft 
altitude above mean sea level (laboratory 
elevation) . A soap-bubble meter was used 
to measure the flows. The mean of 10 
values of bubble transit time was taken 
for each flow determination. The grand 
average flow for a setting of 604 on the 
digital panel meter corresponded to 
661.19 ml/min when corrected to the ref- 
erence temperature and pressure. The 
standard deviation of the flows was 1.95 
ml/min. The reproducibility of the flow 
measurement and setting (the coefficient 
of variation) is 0.3 pet of reading; the 
measured flow values are summarized in 
table 2. 

TABLE 2. - Reproducibility of a flow 
setting, ml/min 



Measured 


Standard 


Flow 


corrected to 


flow 


deviation 


reference temper- 






ature 


and pressure 


661.08 


2.29 




660.65 


660.99 


1.64 




659.12 


658.72 


1.30 




660.57 


657.07 


.32 




660.22 


671.42 


1.86 




664.77 


667.33 


2.82 




661.82 



FLOW CONTROLLER LINEARITY 

We performed our own check of the lin- 
earity of each mass flow controller. The 
control voltage of each mass flow con- 
troller was set at selected values, and 
the corresponding gas flows were measured 
using 10-, 100- , or l,000-ml soap-bubble 
tubes as appropriate, as described ear- 
lier. The flow values and the calibrated 
digital panel meter readings are sum- 
marized in figures 13 through 18. The 
flows were corrected to a flow at our 
reference temperature of 25° C and pres- 
sure of 28.92 in Hg (979.3 mbar). A 
least squares analysis of the data was 



17 




4 6 

DIGITAL PANEL METER READING 



FIGURE 13. - The calibration curve for controller 
1 is the measured controller flow rate corrected to 
25° C, 28.89 in Hg versus scaled digital panel me- 
ter reading of controller output voltage (command 
signal). 




60 80 100 120 140 160 180 2C 

DIGITAL PANEL METER READING 

FIGURE 14. - The calibration curve for controller 
P is the measured controller flow rate corrected to 
25° C, 28.89 in Hg versus scaled digital panel me- 
ter reading of controller output voltage (command 
signal). 



1 1 1 r 



1 r 




60 80 100 120 140 

DIGITAL PANEL METER READING 



180 200 



FIGURE 15. - The calibration curve for controller 
2 is the measured controller flow rate corrected to 
25° C, 28.89 in Hg versus scaled digital panel me- 
ter reading of controller output voltage (command 
signal). 



1,800 


- 


1 


1 


T- 


1 


' 1 1 1 

Air-.^^ 


1 


1,600 


- 










y/ 


- 


1,400 


- 












1,200 


- 










/ y^-^ 


y 


1,000 










/ 


/ z"*^^/^ ^Carbon 
/ yo dioxide 




BOO 








/ 


/ 






600 


_ 




, 


/ 


yy 




_ 








y/ 


.^^ 












/ y'y^ 








400 


" 


/ ^ 


;^^- 








" 


200 




/'^ 


1 




1 


1 , 1 


1 



800 1,200 

DIGITAL PANEL METER READING 



1,600 



2,000 



FIGURE 16. - The calibration curve for controller 
3 is the measured controller flow rate corrected to 
25° C, 28.89 in Hg versus scaled digital panel me- 
ter reading of controller output voltage (command 
signal). 



18 




5,000 



800 1,200 

DIGITAL PANEL METER READING 



1,600 



2,000 



FIGURE 17. - The calibration curve for controller 
4 is the measured controller flow rate corrected to 
25° C, 28.89 in Hg versus scaled digital panel me- 
ter reading of controller output voltage (command 
signal). 



3poo - 



a 2,000 - 



1,000 




1,000 



2,000 3,000 

DIGITAL PANEL METER READING 



5,000 



FIGURE 18. - The calibration curve for controller 
5 is the measured controller flow rate corrected to 
25° C, 28.89 in Hg versus scaled digital panel me- 
ter reading of controller output voltage (command 
signal). 



conducted, and the data are summarized in 
table 3 for the six controllers for air. 
Since the mass flow to volume flow rela- 
tionship depends on the physical (thermo- 
dynamic) properties of the gas , we per- 
formed the linearity characterization 



of controller 2 for pure carbon diox- 
ide, methane, and carbon monoxide. All 
of the data are well characterized as 
straight lines since the square of the 
coefficient of regression approaches 
unity; values obtained for r^ ranged 



TABLE 3. - Summary of the linear regression^ analysis for Tylan 
flow controllers 



Gas 


Controller 


ao 


ai 


r2 


Syx 


Maximum 

flow, 
ml/min 


cv, 

pet 


Air 

Air 

Air 

Air 

Air 

Air 

CO2 

CH4 

CO 


P 
1 
2 
3 
4 
5 
2 
2 
2 


5.37 

.82 

5.44 

78.50 

40.10 

243.60 

3.51 

4.89 

5.83 


0.9824 
1.0410 
.9968 
1.0100 
.9382 
.9505 
.7277 
.7315 
.9973 


0.9999 
.9997 
.9992 
.9999 
.9991 
.9999 
.9986 
.9986 
.9997 


0.739 

.069 

1.840 

7.120 

21.120 

17.350 

1.770 

1.690 

.890 


200 

10 

200 

2,000 

2,000 

5,000 

200 

200 

200 


0.74 
1.40 
1.84 

.71 
2.10 

.69 
1.77 
1.69 

.89 



Regression equation: measured flow = ao "•" ^^ (panel meter read- 



ing) 


and 


ao 
ai 


= 


intercept, 
slope. 












r^ 


= 


coefficient 


of 


regression. 








Syx 


= 


standard deviation of y on 


X, 

^y X 
X 






CV 


= 


coefficient 


of 


variation = 



• 100 pet. 



19 



from 0.9986 to 0.9999. The values of Hq, 
the intercept of the regression line, and 
a^ , the slope of the lines, are given for 
the general equation: 

measured gas flow (in ml/min) 

= ao + a^ (digital panel 

meter reading) . 

Each point is the average of 10 flow mea- 
surements. The average deviation of the 
points from the expected line (Sy^), di- 
vided by the average flow values (one- 
half of maximum flow) , yields coefficient 
of variation (CV) data; the CV values 
ranged from 0.7 to 2 pet for the six 
controllers. 

BINARY MIXTURE PREPARATION 

We used the dynamic gas-mixing system 
to make different concentrations of meth- 
ane gas in air from a standard tank of 
2.58 pet CH4 , and the results are pre- 
sented in figure 19. Two identical flow 
controllers, Nos. 3 and 4, each with a 
range of 40 to 2,000 ml/min, were used to 
generate mixtures flowing at 2,000 ml/ 
min. The methane concentration of each 
mixture was measured by an ANDROS, Inc., 
model 209 methane analyzer, which is 
claimed to have an accuracy and linearity 
of 2 pet of full scale (5 pet methane) , 
i.e., ±0.1 pet methane. The output from 
the analyzer was also measured by a digi- 
tal multimeter (Tektronix DM-501) which 




I 2 3 

METHANE CONCENTRATION MAKEUP, pet 

FIGURE 19. - Measured methane concentration 
versus desired values. Flow controller settings 
were selected using figures 16 and 17 and the de- 
sired flow rates. 

has an accuracy of ±0.1 pet ±2 counts and 
a linearity of ±0.1 pet. 

To prepare the mixtures , we determined 
the dilution ratios and thus the flow 
rates of each component, air and methane. 
Flow rates, read from the calibration 
curves (figs. 16 and 17), were used to 
obtain the appropriate command voltage 
settings. The gas mixture measurements 
are summarized in table 4. 



TABLE 4. - Desired and obtained methane-air mixtures 



Desired 


Desired 


flow. 


Digital 


panel 


Methane 


methane 


ml/min 


meter readings , ml/min 


analyzer 


concentration, 


Standard 


Diluent 


Standard 


Diluent 


readings , 


pet CH4 


gas 


gas 


gas 


gas 


pet CH4 


2.58 


2,000 





1,830 





2.58 


1.29 


1,000 


1,000 


1,035 


1,015 


1.30 


.645 


500 


1,500 


515 


1,570 


.64 


.322 


250 


1,750 


265 


1,850 


.34 


.161 


125 


1,875 


144 


1,998 


.17 


2.064 


1,600 


400 


1,695 


385 


2.09 



20 

A least squares analysis was performed (measured methane concentration, pet) 
on the data from figure 19. The regres- 
sion coefficient was 0.998, and the stan- = 1.01 (makeup methane concentration, 
dard deviation for the data was 0.0143 
for the equation pet) . 

CONCLUSION 

The entire dynamic gas-mixing system confidence in the preparation and use of 

and peripheral components have proved to test gases. It is in continual use and 

be stable, and the dilution of gases has contributes significantly to reducing the 

been very predictable and reproducible. costs of operating the instrument evalu- 

This system has increased our level of ation laboratory. 

FUTURE PLANS 

The dynamic gas-mixing system will be temperature, humidity, gas flow (gas con- 
interfaced to a micro or mini computer as centration) , and time for conducting 
part of an overall automated testing fa- tests of gas monitors for the determina- 
cility for the improved evaluation of gas tion of linearity, accuracy, response 
detection instruments and devices. Soft- time, precision, and stability, 
ware programs will be used to specify 



21 



APPENDIX A.— SUPPLIERS' ADDRESSES 

AADCO, Inc., 2257 Lewis Ave., Rock- Serta Systems, Natick, MS 01760. 
ville, MD 20851. 

Stow Laboratories, Inc. Hudson, MS 
ANDROS Analyzers, Inc., 2332 Fourth 01749. 
St., Berkeley, CA 94710. 

Swaglok, Crawford Fitting Company, 
Forma Scientific, Inc. , Marietta, OH Solon, OH 44139. 
45750. 

Teledyne, Hastings-Raydist , Hampton, VA 
Labcrest, Fischer Porter Co., Warmin- 23661 
ster, PA 18974. 

Tylan Corp., 23301 South Wilmington 
Litronix, Inc., 19000 Homestead Rd., Ave., Carlson, CA 90745. 
Vallco Park, Cupertino, CA 95014. 

Mast Development Co., 2212 East 12th 
St., Davenport, lA 52803. 



22 



APPENDIX B.— DIGITAL TIMER OPERATION AND CONSTRUCTION 



The digital timer is used to manually 
or automatically time soap-bubble flow- 
meter transit times. It is constructed 
from readily available parts, mainly in- 
tegrated circuits and standard light- 
emitting diode (LED) displays. To sim- 
plify flow rate calculations, the display 
is formatted in decimal fractions of min- 
utes. The resolution is 100 ysec and 1 
msec in the 10-min and lOO-min ranges , 
respectively. The 10-min range provides 
a maximum indication of 9.9999 min and is 
usually used for the bubble meter transit 
time measurements. Hour and second time 
bases are also available, with corre- 
sponding resolutions. 

TIME BASE GENERATION AND SELECTION 

All time bases are derived from a 2-MHz 

crystal oscillator (fig. B-1). The 2-MHz 

signal is divided by 2,120, and 7,200 

to produce frequencies of 1 MHz (F^), 

16. 66... kHz (F2), and 277. 7... Hz (F3), 
respectively. 

The desired time base is selected by 
SW-3 (fig. B-2). With SW-3 in the posi- 
tion shown, the control input to select 
gate 2 (pin 4) will float high, and the 
output (pin 6) will toggle at the F2 
rate. The two inputs to both invert 
gates are pulled high, which forces a low 
at the outputs and inhibits select gates 
1 and 3. 

Placing SW-3 in the "Sec" position in- 
hibits select gate 2 (through the cou- 
pling diode) and places a low (ground) on 
the inputs (pins 2 and 3) of the upper 
invert gate in figure B-1. The high out- 
put from this inverter enables select 
gate 1, and allows frequency F^ to appear 
at the output (pin 3) . A similar action 
occurs when SW-3 is in the "Hr" position. 
In this case, select gates 1 and 2 are 
inhibited; select gate 3 is enabled and 
passes F3 to its output. 

The selected time base (F^ , F2 , or F3) 
is buffered and passed on to the input of 
a decade counter and to the range switch, 
SW-4B. The output from the counter (F^/ 



10) is connected to the opposite side of 
the range switch. The output from the 
time base generation and selection sec- 
tion is Fn or Fp/lO, depending on the po- 
sition of the range switch. Switch SW-4A 
sets the decimal point to display one 
(range = 10) or two (range = 100) inte- 
gers in the five-digit display. 

The selected time base is routed 
through one of two run gates in the con- 
trol and gating logic section (fig. B-3) 
to the counters and displays section 
(fig. B-4) . In this section the time 
base is divided by 100 and then fed 
sequentially to five display counters . 
The BCD outputs from the counters are de- 
coded by the 7447 drivers (fig. B-4) into 
seven drive lines to the LED readouts 
(RO). 

TIME BASE CONTROL 

The starting and stopping of the se- 
lected time base frequencies to the coun- 
ters is controlled in two ways: One uses 
a pair of phototransistors, one to start, 
the other to stop the counter; The other 
uses a single switch to start, stop, and 
reset the timer. The circuit for the 
time base control is shown in figure B-3. 
The double-pole switch, SW-2A and SW-2B, 
is used to select either of these two 
ways: In the X position shown, the pho- 
totransistors control the time base to 
the counters and SW-1 is used to reset 
the counters in preparation for another 
measurement; in the Y position, the 
switch SW-1 performs the start, stop, and 
reset operations in succession, thus con- 
trolling the timer as a manually operated 
stopwatch. Each of these time-base- 
controlling methods is described in de- 
tail below. 

Switch SW-1, connected to J-3 (fig. 
B-3) , is used to reset the timer when 
switch SW-2 is set to the X position. 
When momentary contact switch SW-1 is 
closed, a low is coupled through the 
diode to pin 3 of the one-shot multivi- 
brator OS-2. This triggers the one-shot 
multivibrator and produces two 500-psec 



23 



-J-^ 



^ 
^ 



■J-2> 



■J-l> 




^ ^ Control and gating logic 



sw-3 



xo 

2 MHz 



- -rZ 



'J 



Sec A Hr 



? Min 

-^60 -*• 4-60 - 



SW-4a 

10 ^100 

jL Decimal point 
select 



SW-4b ) 



'^ m Counters and displays J 



10 100 



tIO — ' 



LEGEND 
F, I MHz 
Fg 16.66... kHz 
Fj 277.7... Hz 



\^ Time base generation and selection J 

FIGURE B-1. - Functional block diagram of the digital timer. 



7400- 



7400- 



Ik; 



10 




bt used 
.8 



'Select gate I 



SW-3iSec i > 

T'° ^ i ?Hr 1 




Invert 



470 



• i — o — 'W^ • 

point To v+ 

To select 100 
10 




i5iSW-4B To run 
— gates 
I and 2 



^ 2,3,6,7,10 



-Invert 



7-45; 



-/ 



jV* 240 



XTAL 

CSC 
2 MHz 
2N706 



'"^.'^ 14 



-k,. 



*F| 



7492 

-r2 



7492 

-r6 



8 14 



5 



r2 v+ 

5 



7490 
-i-IO 



I 

a 



V+ ♦'^3 



14 



.JM 



10 



7490 
10 



a 



X 



10 



1^7490 



-^6 



FIGURE B-2. = Time base generation and selection schematic. 



24 



IOk< 



>0.068/iF 



>22k 



Y' 220 
SW-2^ o — w^ 



ilL'Oio 



S/S/R 

or 
reset ' t 
SW 



OS- 1 
500^5 






/ 



Reset gate 



y-^ 



10 kJ 



0.068/zF 



— ^h 



lOOuF 

He-- 



FF-fl ^ 



7474 



FF-B 



'2 
7474 



14 



OS- 2 
J 500^5 L 



74121 



SW-2b 



Reset 



- _ A 



lO/iF 



22 k 



300 k: 



Start 

detector o- 

input 



Stop 300 k; 

detector <> ' 

input 



Threshold 
director ^ 



i 

Start gate 

7 13 /_ 



Threshold 
director <^ 



Start 
Stop 
FF 



r 



Reset counters 
to zero 



un gate 




Gated timing pulses 
to counters 

Not usedx 



Run gate 




Selected 
time base 



^Stop gate 
FIGURE B-3. - Control and gating logic schematic. 



To 

FF pin 10 



Fpor Fp/IO 



From<^ 



ilZi I 



5 V+ 



7490 



in 



joT 



> Reset 



v+ 



7490 



D 

P 



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lEA 



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7447 



Reset 



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£Jl 



7490 



7447 



14 II 



7447 



Reset 

oV+ 



5 4 



lEIR 



7490 



14 h 



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5 10112 

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5 4 



II 



"Reset 



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A B C D 
7447 



I9I3I2III0 9 



1 

Reset 



MSD-I 



RO 

Litronix 707 



MSD-2 



D.P 



RO 



From<|> From<|> 



D.R 



RO 



RO 



7 150 a 
7 for 
each 
digit 



RO 

Litronix 707 



3,9,14 



•v,+ 



FIGURE B-4. - Counters and display schematic. 



25 



pulses — a negative going pulse (pin 1), 
and a simultaneous positive going pulse 
(pin 6). The negative pulse goes to pins 
10 and 11 of the stop gate (Start Stop 
FF) and resets the flip-flop (FF) , which 
insures that the run gate (GA-2) is in- 
hibited. The positive pulse (from pin 
6 of OS-2) goes to the seven decade coun- 
ters in the counters and displays section 
and resets the displays to zero. 

When SW-2 is in the Y position, SW-1 
operates as a start, stop, and reset (S/ 
S/R) control. (See fig. B-7 for the tim- 
ing diagram and fig. B-3 for the electri- 
cal schematic.) To explain this opera- 
tion, first assume that the timer has 
just been reset. The A and B outputs 
from flip-flop A and flip-flop B (FF-A 
and FF-B) will be low. The other out- 
puts, pin 6 and B, will be high. The 
low-level signals at the input (pins 1 , 
2, and 13) of the reset gate produce a 
high signal at the output (pin 12) . This 
high is blocked by the diode, and the 
signal has no effect on the reset one 
shot (OS-2). The low (A) signal at pin 3 
of the run gate (GA-1) inhibits this gate 
and does not allow the selected time base 
signal (pin 5) to pass through, (The 
other run gate, GA-2, is inhibited by 
SW-2B . ) 

The first closure of SW-1 (start) trig- 
gers the one shot OS-1, which causes 
flip-flop A (FF-A) to change states at 
the end of the 500-ysec pulse — the A out- 
put goes high and pin 6 goes low. The 
signal from pin 6 has no effect on FF-B, 
(The flip-flops are triggered by a low to 
high transition.) The output (high) from 
the reset gate is not affected because 
pin 2 is held low by the B signal. The 
run gate, however, is enabled because 
both the A and B signals are high. This 
allows the selected time base signal 
to pass through to the counters and 
displays. 

Pressing SW-1 again causes the one 
shot OS-1 to produce another pulse. 
Again, FF-A changes states. The A output 
goes low, and pin 6 goes high. The 



low-to-high transition at pin 11 causes 
FF-B to change states — B goes high, and 
B goes low. The output from the reset 
gate remains high because A (pins 1 and 
13) goes low before B (pin 2) goes high. 
The run gate is inhibited because both 
A and B are low. This stops the passage 
of the selected time base to the counters 
(and display) , and the displays will 
display the accumulated number of se- 
lected time base pulses (or the elapsed 
time) , 

The third closure of SW-1 causes anoth- 
er pulse from OS-1 to trigger flip-flop 
A, which changes states again. Both in- 
puts (pins 1, 2, and 13) to the reset 
gate are high, which causes a low at the 
output. This low, coupled through the 
diode, triggers the one shot OS-2. 

The 500-ysec positive pulse from pin 6 
resets the counters and display to zero. 
The negative pulse (pin 1) is coupled 
through the diode to pins 1 and 13 of the 
flip-flops and resets them to the condi- 
tions described at the beginning of this 
discussion. 

Run gate 2 (GA-2) in the control and 
gating logic section (figs. B-1 and B-3) 
is controlled by two gates which are 
cross-coupled to form a flip-flop (FF). 
The state of this flip-flop is controlled 
by two phototransistors of the soap- 
bubble detector assembly on the flowmeter 
through threshold detectors (fig, B-5) , 
in response to changes in the intensity 
of light falling on the transistors, A 
negative transition (pulse) appearing at 
pin 13 of the flip-flop sets the flip- 
flop producing a high level at the 
output — enabling the run gate (GA-2). A 
negative pulse at pin 9 resets the flip- 
flop and inhibits (disables) GA-2. This 
gates the time base signals through GA-2 
only during the transit time of the bub- 
ble meter bubble. The diagram of the S/ 
S/R sequence is given in figure B-7; fig- 
ures B-5 and B-6 are diagrams of the de- 
tector amplifier and of the power supply, 
respectively. 



26 



+2.5V 
S300k 



lOk 
V\Ar 



10 M 
VvAr 



M 



Start 



2k'' 



10 k 




LED' 



. ^AA/ ^ 



100 




+ 2.5V 



To start gate 
■ GA-1,3 



+ 2.5V 



300 k 

lOk 

i< vAA/ ; ^ 

:^J-2 9V+ 



lOM 



13 



Stop I 



2k 



lOk 




14 



LED' 






IM 



10 



•100 




Ik 



rVW-WV '' 



To stop gate 
■^ GA-I9 



LEGEND +2.5V 

A,_4- LM324 

FIGURE B-5. - Soap bubble transit detection circuitry. 



4 Vdc ^ v + 
display 



V l '' 

-J <l.8k 



5 Vdc ^ v+ 
logic 




► +I2V 



l,OOO^F 



FIGURE B-6. - Timer power supply schematic. 



27 



B 



B 



OS- 1 



H 

L 
H 

L 
H 

L 

H 

L 

H 

L 

H 



r 



I 



OS-2 L 



Start 



1 



J 




¥ 

I 
I 
I 

1 



I 



J 



u \j ^A 



i_r 



Stop 



Reset 



FIGURE B-7. - Timing diagram of logic states of control line of the timer. 



28 



APPENDIX C. — GAS-MIXING SYSTEM PROGRAM FOR TEXAS INSTRUMENTS SR52 CALCULATOR 



Figures C-1 and C-2 contain the program 
for the Texas Instrxjunents SR52 calcula- 
tor. The program was developed to facil- 
itate operation and calibration of the 
system and involves corrections of flow 
rate owing to prevailing temperature and 
pressures. 

The calibration graph for each control- 
ler is a plot of the flow rate at our 
standard temperature and pressure versus 
the digital panel meter reading as de- 
scribed in the paper. The first function 
of the program (User Instructions, steps 
1A-3A) is used to correct the flow rate 
obtained from the calibration graph to 
the prevailing laboratory temperature and 
pressure. 

The second function (User Instructions, 
steps 1B-3B) is the inverse of the first 



and is used to convert a flow rate mea- 
sured under prevailing temperatures and 
pressures to our standard reference tem- 
perature and pressure. 

A soap-bubble meter is used to measure 
the volume flow rate from the controller. 
The average of 10 bubble transit times is 
divided by the bubble meter vol\ime. This 
gives the actual volume flow rate through 
the controller for that particular set- 
ting and indication on the digital panel 
meter at the prevailing temperature and 
pressure. The flow rate is then con- 
verted to a flow rate at STP to be plot- 
ted against the panel meter indication. 
This procedure is repeated at as many 
flow controller settings as required to 
cover the controller range and to produce 
a calibration graph to the desired 
accuracy . 



29 







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BOOKBINDING 

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DEC. 83 







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